1. Field of the Invention
The present invention relates to a coating composition for air bags which firmly bonds to a base cloth, forming a non-tacky cured silicone film thereon. The present invention also relates to an air bag which is fabricated from a base cloth coated with said coating composition.
2. Description of the Prior Art
The air bag is one of the safety devices on the modern automobile. It is located in the steering wheel hub. In the event of an accident, it is inflated between the driver and the steering wheel, thereby saving him from injuries by impact. The air bag is usually fabricated from a nylon cloth coated with chloroprene rubber. Chloroprene rubber, however, does not necessarily have satisfactory heat resistance and weather resistance. This poses a problem associated with the life of the air bag.
The air bag needs to be flame-retardant so that it does not ignite when it is inflated by a pyrotechnic gas. A common practice to remedy the deficiency of chloroprene rubber is to apply a silicone flame-retardant to that part of the chloroprene-coated nylon cloth which is exposed directly to the inflating gas.
The flame-retardant coating is not necessary in the case of an air bag coated with silicone rubber instead of chloprene rubber, because silicone rubber itself can be made flame-retardant by incorporation of a flame retardant. For this reason, silicone rubber is expected to be a coating material for the air bag because of its good heat resistance and weather resistance.
However, silicone rubber is a little poorer than chloroprene rubber in adhesion to a base cloth of polyamide fiber such as nylon. A possible way to eliminate this disadvantage is the pretreatment with a primer, which makes the process complex and aggravates the working environment with organic solvents. Thus, there has been a need for a self-adhesive silicone rubber coating composition which obviates the primer treatment.
A silicone rubber coating composition may be made self-adhesive if it is incorporated with an alkylperoxysilane such as vinyltris(t-butylperoxy)silane, triphenyl(t-butylperoxy)silane, trimethyl(t-butylperoxy)silane, and methyldivinyl(t-butylperoxy)silane, or a silane coupling agent such as vinyltrimethoxysilane, vinyltriethoxysilane, .gamma.-glycidoxypropyltrimethoxysilane, and .gamma.-methacryloxypropyltrimethoxysilane. The silane compound may be replaced by a vinylsilyl group-containing benzoate compound such as t-butylperoxy-4-vinyldimethylsilyl benzoate, which also functions as a vulcanizing agent. However, neither the alkylperoxysilane nor the silane coupling agent provides the self-adhesive silicone rubber composition with sufficient adhesion.
Further, the peroxide-curing type silicone rubber composition poses a problem in the case of hot air vulcanization (HAV method) with an organic peroxide. It remains partly uncured owing to oxygen in the air. Therefore, it gives rise to a tackier thin film than a chloroprene rubber composition. This leads to a possibility that the silicone-coated air bag will not blow up smoothly in the event of an accident.
For these reasons, the silicone rubber coating composition is usually of two-pack type one capable of curing by platinum-catalyzed addition reaction. It has a disadvantage that its curing is easily inhibited by a catalyst poison which might be present in the working environment. In addition, it has a short pot life after the mixing of the two components, which is inconvenient for the continuous operation for a long time. The foregoing has made it necessary to develop a coating composition for the air bag which has a long pot life after the addition of a vulcanizing agent and has no adverse effects on the working environment.